Receivers sometimes include the capability to tune through a number of channels to locate one carrying desired signals. In simple systems, the receiver simply tunes every channel in the spectrum until a channel carrying the desired signals is found. In a more advanced system, disclosed in the above-referenced parent applications in the context of a radio paging system, each transmitter broadcasts a list of alternative channels that are used in the area surrounding the transmitter. On finding one such station, the receiver is provided a small local list of other stations that it can examine for data if the original station is lost, obviating the need to tune through the entire frequency spectrum.
This checking process is energy intensive and reduces the battery life available for the more productive task of receiving pages. If a pager that relies on a local channel list is moved from the geographical area to which the list relates, the pager may exhaust its battery vainly searching for paging signals where there are none. Accordingly, in the system disclosed in the above-referenced applications, after failing to find a paging signal on the channels included on its local list for a predetermined period (37.5 minutes in the illustrated embodiment), the receiver begins a full spectrum scan to find a new paging channel. When it finds a satisfactory paging channel, the pager receives a new local list identifying paging transmitters in that new locality.
While this arrangement greatly reduces battery drain when a pager is moved from one geographical area to another, a period (37.5 minutes) nonetheless elapses during which the pager futilely searches its original (now stale) local channel list looking for paging stations where there are none.
It is an object of the present invention to minimize this futile period, thereby reducing battery drain and increasing message reliability.